Simplified check for duplicate names in select expression list
Introduced unary function templates which use one pair of braces less

This is required for sqlite which cannot parse

SELECT EXISTS((SELECT 1))
Rewrote concat function, preparing for concat and operator||, depending on the connector
Fixed bug in type set
Introduced result::front, pop_front and empty() to make it more STL-container-like
Fixed compile time checks for concat
Rewrote like() member function, added more type control to concat()
Rewrote macro generated function code to handwritten code

This increases the number of Bytes, but it also increases readability,
and reduces complexity.
It also allows to specialize functions for database engines.